If it does and you're using CV mode and your accel could be too slow on one axis (but not the other) to keep up with your feed rate, Mach 3 will round the corners as it's trying to pick up speed to change directions while maintaining your feed rate. If this is indeed what's causing your troubles, you could try the "exact stop on angles greater than" setting and set it to perhaps 89 degrees and see if that fixes it.

It could be what Hood and Peep's posted but also make sure that the radius of the of the tool is smaller or equal to the radius of the corner your trying to cut, else the program may round the corner to suite the cutter.